This invention relates generally to flooring tools, and in particular to cushion back cutters with an internal blade holder design.
A cushion back cutter is a tool for precisely trimming the edges of carpet seams in preparation for making the seams. Cushion back cutters generally take replaceable slotted razor blades to cut through the carpet's backing. They are produced in two designs, which differ primarily in how they hold the blades.
One design is disclosed in U.S. Pat. No. 3,543,401 to Scott. Scott's cushion back cutter has a guide plate 12 with a blade notch 20 in the middle forming a center plate, and has retaining plates 22 forming a left side plate and a right side plate. Guide plate 12 and retaining plates 22 form a blade holding pocket in the area of blade notch 20, and two slotted razor blades left hand blade 24 and right hand blade 26 can be slideably inserted through the bottom of blade notch 20. A screw 32 extends through a retaining plate 22, blade notch 20, slots 28 and 30 of left hand blade 24 and right hand bade 26 respectively, and threads into a nut 34 on the opposite retaining plate 22. When screw 32 is tightened, it clamps left hand blade 24 and right hand blade 26 at a desired extension. Either blade may extend from the carpet engaging edge 14 of guide plate 12 to a cutting position, or else be retracted into blade notch 20 to a storage position. Scott describes how with a left blade 24 extended to a cutting position, the cut made in the carpet's backing “is close to the pile on left side of the space between two adjacent rows of tufts” (col. 3, line 62-64), and that with a right blade 26 extended to a cutting position, the cut made will be close to the pile on the right side (col. 3, line 65-66). As a result, excess backing material is trimmed away, bringing the piles “into abutting relation to conceal the seam” (col. 4, lines 4-5).
The cushion back cutter of Scott has a screw 32 for holding the left hand blade 24 and right hand blade 26 at the desired extension from carpet engaging edge 14 which forms the base of the cutter. This type of cushion back cutter will be referred to herein as an “external blade holder” design because screw 32 is not a feature of the body of the cushion back cutter 10 per se, but is a separate part. If screw 32 is not tightened in nut 34, left hand blade 24 and right hand bade 26 would simply slide out of blade notch 20.
In another design, as shown in U.S. Pat. No. 6,647,628B1 to Braaksma, a cushion back cutter (carpet cutter 10) has a body enclosing left and right internal pockets. The left and right internal pockets each can hold one slotted razor blade 17 only. Guide 12 forms a center plate which has the left and right internal pockets precision machined into its left and right sides. The carpet cutter 10 additionally has left and right side plates attached to the center plate, which in conjunction with the left and right internal pockets formed in guide 12 form blade holding structure 11. When inserted within either left or right internal pockets, a left or right blade may be extended to a cutting position or retracted to a storage position. To trim the carpet's backing close to carpet tufts on the left side of two adjacent rows, the left blade is extended to a cutting position and the right blade is retracted to the storage position. To trim the carpet's backing close on the right side, the right blade is extended to a cutting position, and the left blade is retracted to the storage position.
The cushion back cutter of Braaksma will be referred to herein as an “internal blade holder” design because the blade is held by the left or right internal pockets of the center plate and the left and right side plates forming the body of the cutter. Unlike the cushion back cutter of Scott, the internal blade holder of Braaksma does not use a blade holding screw, but holds the blades by means of the left and internal pockets and side plates.
FIG. 1 shows an exploded view of a prior art cushion back cutter 1 with an internal blade holder design. A center plate 10 includes a left blade pocket 13 (shown with a dashed line) on its left side and a right blade pocket 14 on its right side. Left blade pocket 13 and right blade pocket 14 are machined recesses in center plate 10 which are slightly greater in depth than the thickness of a slotted razor blade. Center plate 10 has a right center plate finger hole 15 through center plate 10 positioned generally at an upper portion of left blade pocket 13. Similarly, center plate 10 also has a left center plate finger hole 16 though center plate 10 positioned generally at an upper portion of right blade pocket 14.
A left side plate 20 and a right side plate 30 are attached to center plate 10, normally by spot welding. Left side plate 20 has a left blade window 25 that is rectangular, and a left side plate finger hole 26. Left side plate finger hole 26 is about the same size as left center plate finger hole 16 in center plate 10, and after spot welding is positioned concentrically with left center plate finger hole 16 of center plate 10. Similarly, right side plate 30 has a right blade window 34 and a right side plate finger hole 35 which is also about the same size as right center plate finger hole 15 in center plate 10. After spot welding, right side plate finger hole 35 is also positioned concentrically with right center plate finger hole 15 in center plate 10.
When left side plate 20 and right side plate 30 are spot welded to center plate 10, a left slotted razor blade 40 is held within left blade pocket 13 of center plate 10 by left side plate 20. Right slotted razor blade 42 is held within right blade pocket 14 in center plate 10 by right side plate 30. Left blade pocket 13 of center plate 10 includes a bottom wall 17, and right blade pocket 14 of center plate 10 includes a bottom wall 18 which control how far left slotted razor blade 40 or right slotted razor blade 42 extend from the base 11 of the prior cushion back cutter 1 when in a cutting position.
After the center plate 10, left side plate 20 and right side plate 30 are spot welded together, they form a cutter body 50. A handle 60 may be fastened to cutter body 50 by passing screw 70 through handle hole 61 and through three cutter body handle holes 51, then tightening with a nut 71.
FIG. 2 shows prior art cushion back cutter 1 with the handle 60 removed from the cutter body 50, a left slotted razor blade 40 inserted within a left blade pocket 13 (both shown with dashed lines) and retracted to a storage position, and right slotted razor blade 42 inserted within a right blade pocket 14 (shown with a dashed line) and extended to a cutting position. In this configuration, right slotted razor blade 42 will trim the carpet's backing close to the right side of the space between two adjacent rows of carpet tufts.
When right slotted razor blade 42 becomes dull, it is changed by use of the forefinger and thumb in a two-step bending and dragging process. First, a forefinger is inserted from the left side of the cutter body 50 through left side plate finger hole 26 in left side plate 20, then through left center plate finger hole 16 in center plate 10, until it contacts right slotted razor blade 42 to bend it outward from the inside. Right slotted razor blade 42 must be bent outward so it can pass over a upper wall 19 of right blade pocket 14. FIG. 5 shows prior art cushion back cutter 1 as right slotted razor blade 42 is being bent outward by pressure from the tip of a forefinger 2 through left side plate finger hole 26 (FIG. 2) and left center plate finger hole 16 (FIG. 2) so right slotted razor blade 42 can pass over upper wall 19 of right blade pocket 14.
Next, as shown in FIG. 2, from the right side of cutter body 50, the thumb can drag right slotted razor blade 42 forward until it contacts a top wall 39 of right blade window 34. From there, right slotted razor blade 42 must be further bent outward, again by the forefinger, so it can pass over the second step formed by top wall 39 of right blade window 34. After right slotted razor blade 42 passes over right blade window top wall 39, it can be removed entirely from right blade window 34 by the using the thumb again to drag it out. The process is similar to remove left slotted razor blade 40 from left blade pocket 13 if it were in a cutting position.
When right slotted razor blade 42 is not in use and needs to be retracted to a storage position, again the tip of the forefinger is used to bend it outward to pass over upper wall 19 of right blade pocket 14. Afterwards, the thumb can drag right slotted razor blade 42 forward until it contacts top wall 39 of right blade window 34. The inside of the right slotted razor blade 42 will rest against the outer surface 12 of center plate 10 and is frictionally held in this position for storage.
From time to time, either slotted razor blade can get stuck in right blade pocket due to debris build up from the carpet being cut, or the gradual dulling of the slotted razor blade, which can cause the cutting edge to mushroom and hang up in the pocket. However, because the finger holes 16, 26 and 15, 35 (FIG. 2) of prior art cushion back cutter 1 are of the same diameter and about the same size as the tip of the forefinger used to bend a slotted razor blade outwards, there is not enough room for the forefinger to move to assist the thumb in dragging the slotted razor blade out of the pocket. The tip of the forefinger simply buts up against the walls of the finger holes and cannot move very far. Furthermore, when a slotted razor blade must be bent outward to a greater degree to pass a top wall of a blade window, more force must be applied to bend the blade further out. Unfortunately, the tip of the forefinger is more or less stuck at the location of the finger holes, and is therefore unable to get additional leverage.
Because the carpet that is being trimmed by the cushion back cutter is very coarse and abrasive, blade change is frequent. Furthermore, in the use of the cutter, the left and right slotted razor blades are adjusted from the storage position to the cutting position on every seam to trim the left side or the right side close to the carpet tufts, adding to the work being done by the forefinger and thumb. A better design for the shape of the finger holes could provide mechanical advantage to the fingers, making blade changes and adjustments faster and easier, and thereby improving the efficiency of the tool.
The figures depict various embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.